In recent years, the precision of the determined current position as determined by a satellite navigation system, such as GPS, has been improving rapidly. The improvement is not only contributed by the improvement in the precision of the GPS receivers but also highly contributed by the use of an augmentation system for the satellite navigation system, such as Multi-functional Transport Satellite (MTSAT) Satellite-based Augmentation System (MSAS).
Nevertheless, the position information provided by a satellite navigation system is still subject to the problem of a position jump, which is an abrupt change in the determined current position of a targeted moving object that may be caused by, for example, an interruption of reception of the signals from the satellites, due to possible interference by trees, buildings or other obstacles. The position jump may be also caused by a multipath effect of the signals from the satellites, a satellite change, or others.
There is also known an inertial navigation system, which is a standalone navigation system using sensors such as accelerometers, gyroscopes, and/or geomagnetic direction sensors, in order to determine the position of a moving object. Inertial navigation systems, however, are subject to a disadvantage in that sensor errors may build up in the process of integration calculation process required for their operations. It would be costly to improve the precision of the sensors, although it may be effective to reduce the sensor errors.
In order to remedy the disadvantages accompanied with the two different types of navigation systems, i.e., the satellite navigation system and the inertial navigation system, there have been proposed various position determination systems utilizing the two different systems so as to generate adjusted position for the current position of the targeted moving object.
Patent publication No. 1 discloses a position determination system of such kind. With this position determination system, it is determined whether or not the positioning process being performed by a GPS receiver is effective. If it is effective, then both the GPS-based determined current position and the inertia-based determined current position are fetched into the system. Then, the difference (or the distance) between a new GPS-based determined current position and the previously GPS-based determined current position is compared with a predetermined threshold. If it is greater than the threshold, the new GPS-based determined current position is deemed as a false position. Also, the difference (or the distance) between a new inertial system-based determined current position and the previous inertial system-based determined current position is compared with a predetermined threshold. If it is greater than the threshold, the new inertial system-based determined current position is deemed as a false position. If the determined current position as determined by one of the two navigation systems is deemed as a false one, the determined current position as determined by the other navigation system is used for navigation purpose. If the difference between the new and the previous determined current positions is smaller than the threshold for each navigation system, one of the newly-determined determined current positions may be selected to be used for navigation purpose, or the average between them may be used for the purpose.
Patent publication No. 2 discloses a navigation method, in which map-matching is performed based on a standalone navigation system. When the map-matching cannot be performed based on the standalone navigation system, then vehicle position data provided by GPS is used to correct vehicle position data derived from the standalone navigation system.
Patent publication No. 3 discloses a solution of the problem of the position jump accompanied with satellite navigation system, in which the solution does not require an aid of any inertial navigation system. According to the solution, it is determined whether or not a position jump has occurred in the newly-provided determined current position of a moving object as determined by the satellite navigation system. If not, then a certain adjustment is effected to the newly-determined current position of the moving object based on an estimated current position of the moving object, and the adjusted position is outputted as an output position. Otherwise, i.e., if it is determined that a position jump has occurred, then, either the estimated current position or the newly-determined current position is used as a new output position.